1. Field of the Invention
The present invention relates to an optical fiber amplifier with an absorber, and more particularly, to an optical fiber amplifier having an absorber for absorbing a signal light having a predetermined wavelength range.
2. Description of the Related Art
An erbium doped fiber amplifier (EDFA) periodically amplifies a weakened optical signal to make up for attenuation of the optical signal due to long distance transmission, when a great amount of data is transmitted over a long distance via an optical fiber. The EDFA directly amplifies light without transforming the light temporarily into electricity for amplification. Thus, the EDFA is effective and economical in amplification.
FIG. 1 shows the structure of a conventional EDFA. The EDFA shown in FIG. 1 comprises a first isolator 100, a first pump light source 102, a first wavelength selective coupler (WSC) 104, an Erbium Doped Fiber (EDF) 106, a second WSC 108, a second pump light source 110, and a second isolator 112.
The operation of the EDFA in FIG. 1 will now be described. First, the first and second pump light sources 102 and 110 inject pump light having a center wavelength of 980 nm. The first and second WSCs 104 and 108 couple the pump light to a signal light having a 1500 nm band which is input via their respective input ports. The pump light excites erbium ions in a ground state in the EDF 106. A signal light is amplified by stimulated emission of excited erbium ions. The amplified signal light is output to the second isolator 112 via the second WSC 108.
The first isolator 100 prevents efficiency of amplification of the signal light from being degraded by amplified spontaneous emission (ASE) from the EDF 106, which is reflected by an optical device, such as a signal input connector, and reenters the EDF 106. Similarly, the second isolator 112 prevents efficiency of amplification from being degraded by the ASE which is reflected by an optical device, such as a signal output connector and reenters the EDF 106.
However, the EDFA has different amplification gains depending on wavelength. In particular, ASE in 1530 nm band is strong, and a gain within this range is high. As a result, the gain in a 1550 nm wavelength band, which is often used for transmission, is reduced and the noise figure increases, thus widening the difference in the gain of the EDFA between wavelengths.
Also, when EDFAs are connected in the middle of a system to amplify an optical signal weakened during transmission, gain is considerably higher in the 1530 nm wavelength band than in other wavelength ranges. Therefore, gain peaking occurs in the 1530 nm wavelength band.